The assembling of various materials such as glasses, crystals, metals, semiconductors, polymers and organic polymeric compounds for optical applications requires particular techniques for ensuring that the resulting assembly preserves a high surface quality as well as optimal optical transmission properties. In some cases, the use of a bonding material such as epoxy or other adhesive materials is to be avoided. Traditional techniques of assembling optical materials may induce mechanical stress, thereby deforming the bonded surfaces, or may result in a bonding having insufficient mechanical strength. In addition, the bonded surfaces may present local or extended damages resulting from the bonding techniques themselves. Moreover, due to the presence of visible bonding marks and residual surface deformations, these techniques may also alter the optical properties of the resulting assembly, such as its transparency or its reflectivity. Therefore, new approaches to the assembling of optical materials need to be developed in order to provide bonded structures that retain high optical qualities and remain exempt of alterations and defects induced by the bonding processes.
It is known that two solid materials of virtually any composition may be joined together using the method of direct bonding. Direct bonding relies on molecular bonding forces occurring under specific conditions at the interface between two surfaces. When two polished surfaces are brought close enough to each other, intermolecular van der Waals forces become sufficiently strong to maintain together the surfaces without any other bonding means. Hence, in the case of optical components, the optical quality of such direct bond is extremely high since no deformation, damage or other mechanical stress is induced. Furthermore, the optical properties of the bonded materials are preserved since direct bonding does not require or induce any physical or chemical alteration.
Other techniques have been developed for joining together two surfaces. For example, the use of ultrashort laser pulses to join transparent materials is a known technique. The ultrashort pulse filamentation effect in transparent materials creates a linear plasma column arising from the non-linear optical process of self-focusing. The overall length of this plasma column is proportional to various parameters, including the focal length of the focusing objective and the incident pulse energy. This approach has been proposed by Itoh et al. in U.S. patent application no. 2010/0047587, where it is stated that the generation of relatively long filaments (i.e. filaments longer than 100 micrometers) permits the inscription of laser weld seams between two transparent dielectric materials. One advantage of this procedure is that it is not overly sensitive to the positioning of the axial focal spot of the laser pulses. However, this method requires excessive pulse energy in comparison to what is really needed to induce non-linear absorption. The long filaments thus generated modify matter in a region extending from several tens to hundreds of micrometers inside each material, which is much larger than the thickness of the interface. The strong non-linear absorption may also cause unwanted damage mechanisms. In U.S. patent application no. 2007/0051706, Bovatsek et al. propose the use of an ultrashort pulse train at high repetition rate to bond together two surfaces in a process that deforms these surfaces. In this approach, the surfaces to be bonded are first locally deformed using high-power ultrashort laser pulses so as to locally bring closer (or raise) these surfaces. A second pass of the laser is then used to form the weld between the surfaces. This approach can however degrade the surface quality of the bonded materials.
Furthermore, several traditional methods of laser welding require that the surfaces be maintained together mechanically during the welding process. This may be accomplished, for example, with the use of a clamp or an air jet, which applies a sufficient pressure on the surfaces during exposure to laser pulses. In another similar method, the two materials are brought into contact under relatively high pressure and for a sufficiently long period of time, so as to temporarily bond the surfaces after withdrawal of the clamp or other device, in a process related to cold welding. These methods rely on elastic deformation of the materials, in which important residual stress build-up and surface deformations may reduce the mechanical strength of the resulting bonded structure.
Laser sealing of a direct bond was proposed by Haisman et al. in U.S. Pat. No. 5,009,689 but its applications are limited by the use of a continuous laser beam. Therefore, this process cannot be utilized when the two materials to be joined together are both transparent to the wavelength of the laser (e.g. glasses), irrespectively of whether these two materials are identical or dissimilar. Further, the process proposed by Haisman et al. relies on linear absorption of the laser energy by one of the two materials, thus creating local fusion by purely thermal mechanisms. In order for the process to operate successfully, a bond activating treatment is necessary, thereby adding an additional preparation step for the surfaces to be joined.
It has been proposed by Miyamoto et al. (I. Miyamoto, K. Cvecek, Y. Okamoto and M. Schmidt, “Novel fusion welding technology of glass using ultrashort pulse lasers”, Physics Procedia, vol. 5, 2010, pp. 483-493) to weld glass plates, pre-assembled by optical bonding, with 10-picosecond laser pulses emitted at high repetition rate. In this laser welding regime, adverse thermal effects can induce localized damages and defects, such as cracks, that affect the optical properties of the assembly. An extensive review of the damages and defects arising in this laser welding technique has been presented by Cvecek et al. (K. Cvecek, I. Alexeev, I. Miyamoto and M. Schmidt, “Defect formation in glass welding by means of ultra short laser pulses”, Physics Procedia, vol. 5, 2010, pp. 495-502).
Finally, in U.S. patent application no. 2010/0304151, Tuennermann et al. describe a method for laser-assisted bonding of substrates, in which the substrates are connected together firstly frictionally by pressing together and thereby achieving a state of optical contact. Subsequent strengthening of the connection is effected by activation in regions which is induced by an ultrashort pulsed laser. This activation results in the local heating of the exposed region without reaching the melting point of the materials, so that this process of laser-assisted bonding departs from the traditional scope of welding. Moreover, there is no mention of an unaltered optical transmission window or of any other strategy by which the optical properties of the resulting assembly remain unaltered following the process of laser irradiation.
In view of the above considerations, there is therefore a need for a method for joining together optical components that provides high surface and optical qualities while alleviating at least some of the drawbacks of the prior art.